RF tagging systems are well known in the art. In one such system, the existence of a single resonant circuit in a detection field or zone is utilized as an anti-theft type apparatus. Essentially, if an article having a single resonant frequency tag passes through a detection zone, an alarm is generated which indicates the unauthorized presence of store goods in the detection zone. Such resonant circuits have been constructed in accordance with standard printed circuit board techniques.
Some prior RF tagging systems have provided multiple different tuned (resonant) circuits on a tag so as to specifically identify the goods to which the tag is attached or the destination to which those goods should be directed. Such systems have been proposed for parcel or other article delivery systems wherein resonant circuits are utilized to provide a destination or sender code rather than printed bar codes.
The use of resonant circuit tagging is advantageous in that it is not subject to problems such as dirt obscuring a portion of a printed bar code and causing an error in determining the code associated with the article. Also, exact alignment of the tag with the detection system may not be required in RF tagging systems, since generally it is desired only to detect the presence of the resonant circuits somewhere in a broad detection zone. This can be achieved without precise alignment between the resonant circuit, the detection zone and the detection apparatus.
In order to enhance the utility of such systems, RF tags having multiple resonant circuits have been proposed to thereby increase the number of possible different identification codes. Prior systems utilizing multiple tuned circuit detection contemplate sequentially generating or gating each of the different resonant frequency signals to a transmitter antenna. Each different resonant frequency in a multiple frequency system is provided by a master oscillator circuit or transmitter whose output is essentially swept or stepped to sequentially provide each desired output frequency. Then, reflected energy from each of the tuned circuits is detected. Some frequency tagging systems look for absorption of RF energy by the resonant circuits during the transmission of the test frequency signals.
An improved RF tagging system is fully described in copending application Ser. No. 07/966,653, filed on Oct. 26, 1992, in the names of Sanjar Ghaem, Rudyard L. Istvan, and George L. Lauro, for RF Tagging System and RF Tags and Method, which application is assigned to the assignee of the present invention and fully incorporated herein by reference. The system there disclosed includes, as a significant feature, the simultaneous radiation of RF energy at a plurality of different frequencies in order to detect each of a plurality of different frequency resonant circuits which may be provided on a tag. Then a code signal indicative of which resonant frequencies for the tag resonant circuits were detected is provided. The above feature results in very fast detection of which resonant frequency circuits are provided on a tag in a detection zone.
The provision of multiple resonant circuits on RF tags, while providing a multiplicity of possible different identification codes, has many disadvantages. For example, multiple resonant circuits can occupy considerable RF tag surface area. This is especially the case with resonant circuits formed in a common plane. Hence, as the number of RF resonant circuits is increased, the tag size must also increase. As a result, the utility of prior tagging systems has been limited to those applications wherein large tags may be accommodated or where relatively few resonant circuits are needed on a tag.
Another disadvantage of tagging systems requiring multiple resonant circuits on each tag is in programming each tag to provide a particular identification code. Some prior RF tagging systems which contemplate printing a large number of different resonant frequency circuits on a tag create different codes by the selective adjustment of some of these resonant circuits. These systems have recognized that it may be necessary to adjust the resonant frequency provided for each circuit and such adjustment is generally contemplated as occurring by selective removal of metalization forming the resonant circuit. Some systems have recognized that step adjustments of the resonant frequency of such tuned circuits is desirable and this has been implemented by punching holes of predetermined diameters in capacitive elements of the resonant circuit to thereby reduce capacitance and increase the frequency of the resonant circuit. Such known prior techniques are not readily adaptable to mass production of customized resonant frequency codes by a post factory manufacturing operation. Many times, the actual code to be utilized will not be known until immediately prior to attaching a tag or label to an article.
A different approach to provide RF tag identification codes utilizes a single resonant circuit on an RF tag which is resonant at a single frequency. When the identification code of the RF tag is read, a transmitter continually illuminates the tag with RF energy at the single frequency. Electronic circuitry on the tag selectively renders the resonant circuit reflective and nonreflective in accordance with a time function. A reader monitors the reflected energy and decodes the time function to recover the identification code. This approach is also limited in the number of possible identification codes for a given read time because only a single resonant frequency is utilized.
Hence, there is a need in the art for an improved RF tag and RF tagging system utilizing the same which provide a greater number of potential identification codes than heretofore possible within given read time constraints. Such a system should provide the greater number of identification codes without requiring an increased number of resonant circuits on the RF tags. Further, the RF tags of the improved system should be arranged for ready programming of the identification codes immediately prior to being associated with their respective goods or articles. Still further, such an improved system should be capable of being implemented with known and commercially acceptable technology and techniques to render the improved system commercially advantageous.